Is it Cheating?

So, here's the scenario. My girlfriend, her friend and I are all sitting in the back of this car. My best friend and another friend are sitting up front, we are all drinking, having a good time. The girls are GONE, and i step out for a bit to get some fresh air. I'm gone for...3 minutes? I come back, I find out my girlfriend made out with her friend, after her friend kissed her. My friends up front swear it was for a good minute, it wasn't a peck, it wasn't that she was in shock, she seemed to go with it, for a good solid minute.

Things to consider:

1. She is a pansexual (my girlfriend).
2. Anyone guy who comes in here saying I should stop worrying because it's hot, stop. I don't care, simply because it's a stupid stereotype to put on girls.
3. When I asked her if it was then okay to make out with a straight girl-friend of mine, she said no it's not okay, why? Because she's straight and so am I.

If making out/kissing with someone else is cheating to you in your opinion, then yes.

"Reminds me of a dog. You stare at it while it's playing outside, running around in a circle, then stopping up, wagging with a wide "grin", jumping around until you call for the dog. Then it lowers its tail because it think it's done something bad and then you throw the ball, seeing the tail go straight into the air in bliss! ^^" - Gehco

I would consider it cheating, the fact that she did it while you were around means she probably thought it was quite innocent though.

I think it's important that you make it clear that for you it's not ok. In a mature relationship (assuming monogamy was a mutual understanding) that's all that matters, if that's a problem then there is a problem.